1. Field of the Invention
The invention relates to a process for producing polychrome ceramic shaped parts, to polychrome ceramic shaped parts themselves and unfired ceramic shaped parts.
2. Description of the Related Arts
Colored ceramic shaped parts can be produced by sintering a granulate. Color-producing particles are here mixed with the ceramic powder and, if desired, sintering aids to give a granulate. For the purposes of the present invention, color-producing particles are particles which have the desired color before sintering or develop the desired color only during the sintering process. The components are mixed by a milling process or by precipitating-on from a solution. In both cases, the mixture subsequently has to be dried, in the case of precipitating-on of a solution it is frequently necessary to calcine the powder mixture. Shaping can be carried out by cold or hot isostatic pressing. Sintering then gives a single-colored ceramic shaped part. These two methods of coloration are technically complicated.
A specific process for preparing ruby red aluminum oxide ceramic is described in the Czechoslovakian Patent CS 191 122. The granulate here comprises a mixture of aluminum oxide ceramic powder and a binder. After mixing, this granulate is injection molded. The shaped parts are colored by impregnation with a chromium chloride solution after partial removal of the binder. The impregnated shaped part is subsequently sintered. However, it is a disadvantage that demixing phenomena occur during impregnation and subsequent drying, because the chromium chloride is predominantly absorbed on the surface, although the solvent fully penetrates into the shaped part. A homogeneous color distribution cannot be achieved using this method.
Owing to the difficulties associated with just single-colored ceramic shaped parts, it has hitherto not been possible to produce polychrome ceramic shaped parts. The only known method is the mixing in of a number of different color-producing particles to give a granulate, so that in the case of non-mixable colorants uncontrollably speckled ceramic shaped bodies can be formed.
It is an object of the present invention to provide a process for producing polychrome ceramic shaped parts having good coloration and color distribution properties. In particular, the production of a defined interface between the color phases should be made possible. This process should be simple and flexible. A further object of the invention is the provision of polychrome ceramic shaped parts and of unfired ceramic shaped parts.